1. Field of the Invention
The present invention relates to a noise canceling system, and more particularly to a noise canceling system which cancels a plurality of background noises that infiltrate into a voice receiver through different transmission paths.
2. Description of the Prior Art
The common noise canceling system for removing (canceling) from the output of the voice receiver noises generated from a plurality of noise sources and received by the voice receiver is such that the frequency transmission characteristics such as impulse response and transmission functions of noise transmission paths from the noise sources to the voice receiver, are estimated, and the noises are produced via the estimated frequency transmission characteristics, linearly added up together, and are subtracted from the output of the voice signal receiver so as to be canceled.
According to the above-mentioned conventional noise canceling system, however, the amount of operation becomes essentially very great.
That is, in the above typical noise canceling system, frequency transmission characteristics of noise transmission paths from noise sources to a voice receiver are estimated by some means, filters such as transversal digital filters having transmission functions that offer the above frequency transmission characteristics are constituted as equivalent noise-producing filters, and noises generated by the noise sources are produced via the equivalent noise-producing filters, added up together linearly, and are subtracted as an equivalent superposed noise of the plurality of noise sources from the output of the voice receiver so as to be canceled. Therefore, how efficiently to estimate the coefficients of transversal filters that constitute an equivalent noise-producing filter, is very important for preventing the amount of processing from greatly increasing.
The filter coefficient of such an equivalent noise-producing filter is estimated as described below. That is, when there exists a single noise source, the filter coefficient which minimizes the electric power of noise-canceled residual waves after the output of the transversal filter is subtracted from the output of the voice receiver, is determined by widely known methods such as solving an inverse matrix of a row number and a column number determined by the tap number of the filter or searching relying upon a maximum inclination method. Where there exist a plurality of noise sources, the coefficients of a plurality of equivalent noise-producing filters must be determined by taking the effects among the noise sources into consideration. Even when there exists only one noise source, however, the amount of processing and operation becomes essentially very great. The amount of processing and operation becomes tremendously great when a plurality of noise sources have to be treated by giving attention to the effects among the noise sources.
According to another method for estimating the filter coefficient of the equivalent noise-producing filter, the filter coefficient which minimizes the electric power of noise-canceled residual waves, is set over a considerably long period of observation time by forming an automatic control loop and by effecting the adaptive control. However, since the observation time is considerably long, the processing response tends to be considerably delayed even when there exists only one noise source. In particular, this method exhibits poor follow-up performance for the noise that changes with time.